warnless: drop parts of the read/write preprocessor hack (Windows)

The `#undef` hack is no longer necessary after changing the redifitions
to not map back to the original symbols.

This makes it unnecessary to repeat the redefinitions after compiling
`warnless.c` itself (in unity mode).

Which in turns makes it unnecessary to include `warnless.h` again, to
trigger such redefinition.

This also means that `read`/`write` are now redefined on Windows from
the first inclusion of `warnless.h`.

Also:
- tests/server: drop a repeat `warnless.h` include, that is unnecessary
  after this patch.
- tests/unit: drop repeat `warnless.h` include.
- tests/libtest: drop repeat `warnless.h` includes.
- tests/libtest: formatting.

Follow-up to 2f312a14da #17619
Follow-up to 84338c4de2 #12331
Follow-up to 6239146e93

Closes #17673

tests/unit/unit1309.c

@@ -24,7 +24,6 @@
 #include "curlcheck.h"
 
 #include "splay.h"
-#include "warnless.h"
 
 static void splayprint(struct Curl_tree *t, int d, char output)
 {